Hi,
i'm trying to map this RSS:

<rss version="2.0" xmlns:media="http://search.yahoo.com/mrss">
    <channel>
        <title>Dailymotion</title>
        <link>http://.....com</link>
        <description>Random video</description>
        <lastBuildDate>Tue, 27 Oct 2009 17:43:53 +0100</lastBuildDate>
        <item>

            <title><![CDATA[I giochi che fanno diventare]]></title>
            <link><![CDATA[http://..../video_download.php?guid=/video/xb3akf_i-giochi-che-fanno-diventare-_fun]]></link>
            <guid><![CDATA[http://..../video_download.php?guid=/video/xb3akf_i-giochi-che-fanno-diventare-_fun]]></guid>
            <description><![CDATA[Una classifica dei 10 giochi che
hanno condizionato le scelte di un'intera
generazione.]]></description>
            <pubDate>Tue, 10 Nov 2009 09:54:52 +0100</pubDate>
            <media:thumbnail
url="http://..../proxyimg.s4af92a5c19cb1/21565/image.gif" height="91"
width="120" />
        </item>
        ....
       </channel>
</rss>


I don't know how to map the "media:thumbbail" value.
I tryed in this way:

<binding>
<mapping name="rss"
class=".........Rss" ordered="false" >
<namespace prefix="media" uri="http://search.yahoo.com/mrss" default="all" />
<value name="version" style="attribute" field="version"  usage="optional"/>
<structure name="channel" field="channel" type=".........Channel"
ordered="false">
<value name="title" field="title"  usage="optional" />
<value name="link" field="link"  usage="optional" />
<value name="description" field="description"  usage="optional" />
<value name="lastBuildDate" field="lastBuildDate"  usage="optional" />
<collection field="items"
factory=".........Channel.itemsListFactory" usage="optional">
<structure name="item" type=".........Item"  ordered="false"
flexible="true">
<value name="title" field="title"  usage="optional" />
<value name="link" field="link"  usage="optional" />
<value name="guid" field="guid"  usage="optional" />
<value name="description" field="description"  usage="optional" />
<value name="pubDate" field="pubDate"  usage="optional" />
<structure name="thumbnail" type=".........Thumbnail"
ordered="false" flexible="true">
<value name="url" style="attribute" field="url"  usage="optional"/>
<value name="height" style="attribute" field="height"
usage="optional"/>
<value name="width" style="attribute" field="width"  usage="optional"/>
</structure>
</structure>
</collection>
</structure>
</mapping>
</binding>

But i receive this error ar runtime:

No unmarshaller for element "rss"

Thanks
Davide